Process for producing pure phosphates



-20 crystals one or more times.

'25 titles of salts in impure mother liquor.

Patented Oct. 10, 1933 UNITED STATES PATENT OFFICE PROCESSFOR PRODUCINGPURE PHOSPHATES f Charles F. Booth, Anniston, Ala., assignor to SwannResearch, Incorporated, a corporation of Alabama No Drawing. ApplicationDecember 16, 1931 Serial No. 581,516

This invention relates to a process of producing pure ortho-phosphates.

One object of this invention is the provision of a process for theproduction of pure solutions of alkali metal or ammoniumortho-phosphates from solutions containing impurities particularly ironand aluminum.

Another object of this invention is the provision of a process for theproduction oithe ordinary alkali metal phosphates such as the monoordi-sodium or potassium ortho-phosphates or the monoor di-ammoniumortho-phosphates in a pure form. Pure ortho-phosphates are considered tocome within the object of this invention '15- when they produce littleor no turbidity when dissolved in water. 7

Previously known methods of producing the alkali metal or ammoniumphosphates in relatively pure form involve recrystallization of the Sucha procedure is exceedingly undesirable from the standpoint of cost,occasioned not only by the expense of these operations, but also byreason of the low yield of pure salt due to the accumulation of largequanmono-basic alkali metal or the mono-ammonium ortho-phosphates, beingsomewhat acid in solution, are furthermore diflicult to purify of con-'-tained iron and aluminum compounds since these compounds do notprecipitate completely unless the pH of the solution is considerablygreater,

i. e., the solution is more alkaline than that of the correspondingmono-basic salt solutions.' Even under conditions of greater alkalinityit is difii- 5 cult to remove the impurities such as ironor aluminumcompounds completely.

I have now found that by treating the monobasic ortho-phosphate solutionwith a manganese compound in which the manganese has an average valenceof three, I am enabled to remove any associated impurities such as ironor alumina or both completely and at very little cost. Such re- Thecompound to the trivalent form.

Having prepared a pure mono-basic ortho-,

9 Claims. (01. 23-107) monium phosphate. It will, of course, beunderstood that itmay be applied in like manner to the production ofalkali metal phosphates, without departing from the spirit of the hereinclaimed invention. 7

To ortho-phosphoric acid containing say 0.35% or" iron and aluminumphosphatesfadd ammonia until a 5 cubic centimeter. sample requires 0.5to

1.0 c; c. of ga cid for titration to the methyl orange end point. Addsufiicient water so that the specific gravity of the solution is in theneighborhood of 1.18 at 25 C. I now add to the solu tion a manganesecompound in the trivalent state of oxidation to efiect precipitation.

Since the'trivalent manganese, whichI employ in effecting precipitationof theiron and alu minum compounds, is not readily availablecommercially, I prefer to prepare it directly in the solution to betreated. This may be=done in either one of several ways as suggestedbelow under a, b, or c.v V

(a) As a source of divalent manganese, I may employ the readilyavailable commercial grade of manganese sulphate (MnSOi/lHzO).Sufiicient of this salt is added to the ammonium phosphate solution asprepared above to'give from 0.013 to 0.105% of manganese in thesolution; The divalent manganesefis now brought to the next higher stateof oxidation by the'addition of potassium permanganate. 'Usually from0.572

to 0.645 pounds of KMnO4 per pound of manganese presentin the solutionis required. The addition of this 'compound will oxidize the divalentmanganese to the trivalent form, at the same time being form.

(12) Starting with tetra-valent manganese, I may employ the readilyavailable manganese di-' oxide. This compound isadded directly to theabove mentioned ammonium phosphate solution, together with sufiicientoxalic acid to reduce this In order to effect this reaction the solutionis boiled.

(0) Starting with hepta-valent manganese IL itself reduced to thetrivalent may employ ordinary potassium permanganate,

adding sufficient oxalic acid to reduce the manganese to the trivalentform. I

The solution treated by either of the three methods outlined above isnow boiled to effect complete precipitation of the impurities and theadded manganese compounds. It is then cooled and clarified by filtrationor decantation. If the solution is somewhat pink due to the presence ofsome colored manganese compound, as is sometimes the case, I preferablydischarge this by the addition of a suiiicient quantity of oxalic acid.In order to crystallize mono-ammonium phosphate from solution it ispreferably concentrated by evaporation to a specific'gravity of 1:36 at100 C.', after whichit is again cooled. The separated crystals areremoved in the ordinary manner with a centrifuge, the mother liquorbeing further concentrated by evaporation, or returned j to thebeginning of the cycle.,,,Although I have described my process asemploying a manganese compond in which this compound has a valence ofthree, it will be understood that I am undoubtedly working with amixture of compounds in which the average valence of the manganese isthree.

also in a lower state of oxidation. If, however, a mixture of compoundshaving an average valence of three is employed, it will be foundto beefiective for the purpose claimed.

" 'I' may of course prepare diammonium' phos- 'phate by the furtheraddition of ammonia to the pure concentrated mono-ammonium phosphatesolution obtained as above. Crystallization of this product is effectedin known manner.

While I have described in detail one method for treating an ammoniumphosphate prepared from an acid originally. containing in theneighborhood of 0.35% iron and aluminum phosphate, my method may alsobeapplied to solutions of phos-' phates prepared from acid havinggreater or lesser amounts of these impurities, by varying M somewhat theproportion'sjof trivalent manganese employed. Where greater amounts ofthese impurities are present a greater quantity of manganese is requiredand vice versa.

My method is also applicable to the purification m of solutions of othermono-basic phosphates such 40 i from said aqueous solutions.

2. Process of producing pure ammonium orthophosphates from aqueoussolutions containing ammonium ortho-phosphates associated withimpurities comprising forming in solutions of monoammonium,ortho-phosphate a trivalent manganese compound and thereby precipitatingsaid impurities from said aqueous solutions.

3. Process of producing pure ammonium orthophosphate from aqueoussolutions containing ammonium ortho-phosphate associated withiron andaluminum compounds comprising forming a As is usual in solutions of-thistype, some manganese will be present in a higher and trivalent manganesecompound in said solution, precipitating said iron and aluminumcompounds while the ortho-phosphate is in the monobasic stage and thenrecovering ammonium orthophosphate from said solution.

4. Process ofproducing pure mono-ammonium ortho-phosphate from aqueoussolutions containing said mono-ammonium ortho-phosphate together withiron and aluminum compounds comprising forming a trivalent manganesecompound in said solution, precipitating said iron and aluminumcompounds, clarifying said solutions 1 and recovering saidorthophosphate from solution.

5. Process of producing mono-sodium orthophosphate from aqueoussolutions containing said ortho-phosphate together with iron andaluminum compounds comprising forming a trivalent manganese compound insaid solution precipitating said iron and aluminum compounds, clarifyingsaid solution and recovering said sodium orthophosphate from solution. ea

6. In the process of producing pure alkali-metal or ammoniumorthophosphates, the step of purifying aqueous solutions of monobasicorthophosphates containing iron and aluminum as impurities, comprisingforming in said aqueous solutions 2. dissolved trivalent manganesecompound and precipitating thereby said iron and aluminum compounds.

7'. In the process of producing pure alkali-metal or ammoniumorthophosphates, the step of purifying aqueous solutions of monobasicorthophosphates containing iron and aluminum compounds as impurities,comprising adding to said aqueous solution a divalent manganese salt andoxidizing said divalent salt to the trivalent stage and precipitatingthereby said iron and aluminum compounds.

8. In the process of producing pure alkali-metal or ammoniumorthophosphates, the step of puritying aqueous solutions of monobasicorthophosphates containing iron and aluminum compounds ascimpurities,comprising adding to said aqueous solutions a tetravalent manganesecompound, reducing said tetravalent manganese to the trivalent form andprecipitating thereby said iron and aluminum compounds from saidsolution.

I 9 In the process of producing pure alkali-metal or ammoniumorthophosphates, the step of purifying aqueous solutionsof monobasicorthophosphates containing iron and aluminum compounds as impurities,comprising adding to said aqueous solution a heptavalent manganesecompound, re-

ducingsaid heptavalent manganese to the trivalent form by means ofoxalic acid, and precipitating said iron and aluminum compounds fromsolution.

CHARLES F. BOOTH.

